(1) Field of Invention
The present invention relates in general to a liquid crystal display, and more particularly, to a liquid crystal display having a repair line.
(2) Description of Related Art
A thin film transistor TFT liquid crystal display LCD is a display which uses a TFT for a switching element and uses the electro-optical effect of a liquid crystal material to create an image. Such an LCD, for each pixel, includes a TFT and a pixel electrode, a gate line to provide a switching signal to the pixel, a TFT substrate in which a data line is formed to provide a picture signal, a color filter substrate in which a common electrode and color filter are formed, and a liquid crystal material intervened between them.
Gate lines and data lines providing signal paths to a pixel are formed in the TFT substrate of an LCD, but are apt to be broken in the process of creating product. If such lines are broken, the LCD may not perform properly as a display.
One conventional method of solving this problem is to use a repair line in place of a broken wire when the break occurs after forming an enclosed curve type repair line around a screen formed of pixels. This intersects with gate lines and data lines with a gate insulating layer between them.
Reference will now be made in detail to a conventional LCD, an example of which is illustrated in the accompanying drawings.
FIG. 1 is a plan view illustrating a conventional LCD wherein a repair line is formed as an enclosed curve around a screen, FIG. 2 is an equivalent circuit diagram of a general pixel. FIG. 3 is a plan view illustrating the layout of the lower substrate in FIG. 2. FIG. 4 is a sectional view illustrating a section cut along A--A line in FIG. 3. FIG. 5 is a diagram illustrating in greater detail portion 6 of FIG. 1, and FIG. 6 is a sectional view illustrating a section along the B--B line of FIG. 5.
In a general LCD, as illustrated in FIG. 1, gate lines G.sub.1, G.sub.2, . . . G.sub.m are formed in parallel and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n intersecting perpendicular to them are formed.
Input pads 1, 1', 2 are formed on each row of data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n and gate lines G.sub.1, G.sub.2, . . . , G.sub.m. The data lines may be divided into several first data lines D.sub.1, D.sub.3, . . . D.sub.2n-1 in which each input pad 1 is formed in the upper part of the LCD and second data lines D.sub.2, D.sub.4, . . . D.sub.2n in which each input pad 1' is formed in the lower part of the LCD. Pixels 10 are formed in the spaces at which the gate lines G.sub.1, G.sub.2, . . . , G.sub.m and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n meet.
As illustrated in FIG. 2, each pixel 10 includes a switching element, generally TFT 20, a liquid crystal storage capacitor 13, and a holding capacitor 14. Liquid crystal storage capacitor 13 is formed by a lower pixel electrode 12, an upper common electrode 11 and liquid crystal material between them. Such a pixel 10 is connected to one data line and one gate line through TFT 20.
A lay-out of the upper portion of the pixel electrode and a section of TFT 20 corresponding to the upper part of a pixel having a structure as in FIG. 2 is illustrated in FIGS. 3 and 4.
On a transparent insulating substrate 21, a gate line G and a gate electrode 22 which are connected to it are formed. A gate oxide film 23 is formed over each gate line G and gate electrode 22. A gate insulation layer 24 covers oxide film 23, and substrate 21. Over the gate insulation layer 24, at the portion covering the gate electrode 22, is formed a semiconductor layer 25. Generally, on the semiconductor layer, which is typically formed of amorphous silicon, two divided contact layers 26 are formed, typically of n.sup.+ amorphous silicon. A source electrode 27 of the TFT connects to the data line D and covers one of the contact layers 26. A drain electrode 28 separated from the source electrode 27 and connected to the pixel electrode 12 covers the other contact layer 26. The contact layers serve to improve the electric connection from the semiconductor layer 25 to the source and drain electrodes 27, 28.
FIG. 1 illustrates a repair line 100 formed in a enclosed curve around a screen 3 formed of pixels 10. Such a repair line 100 intersects with each gate line G.sub.1, G.sub.2, . . . , G.sub.m around a input pad 2 and each data line D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n a first time around the input pad 1, 1' and a second time at the opposite end of the data line. Each data line D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n has the same width throughout. Also, data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n and repair lines have a same intersecting area respectively.
Reference will be made to FIGS. 5 and 6, which illustrates the intersecting of data lines D.sub.2n-1, D.sub.2n and a repair line 100.
As illustrated in FIGS. 5 and 6, a first data line D.sub.2n-1 and a second data line D.sub.2n has a same intersecting area with a repair line 100. If considering FIG. 6 as a cross section along the B--B line of FIG. 5, a gate insulation layer 24 formed on the repair line 100 insulates repair line 100 from the data lines D.sub.2n-1, D.sub.2n. A protective film 29 is formed on the data lines D.sub.2n-1, D.sub.2n. Accordingly, the intersecting area between the data lines D.sub.2n-1, D.sub.2n and the repair line 100 acts as an undesired capacitor and parallel connected capacitors result.
During operation of an LCD as described above, if a switching signal is applied to each pixel 10 in turn through gate lines G.sub.1, G.sub.2, . . . , G.sub.m, a picture signal is applied to the corresponding pixel 10 through data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n and provides an image in accordance with it. However, if a data line such as D.sub.3 is disconnected as illustrated in FIG. 1, the picture signal applied through the data line D.sub.3 reaches the disconnected point but does not reach the next point. Thus, this picture signal cannot be applied to a pixel connected to the data line D.sub.3 below the break or disconnect point illustrated with the symbol "//".
If the data line D.sub.3 is repaired using a repair line 100, the signal reaches the data line D.sub.3 below the disconnected point. Referring to FIG. 1, the area indicated with .DELTA., where the data line D.sub.3 and the repair line 100 intersect, is short-circuited using a laser. A signal applied from a input pad 1 in a pixel connected to the data line D.sub.3 below the disconnected point, after passing through the upper of the disconnected intersecting area, moves along the left path of the data line D.sub.3 through the repair line 100 connected with the data line D.sub.3 (path P1) or along the right path (path P.sub.2). However, because the path P.sub.2 is longer than the path P.sub.1 and intersects with many data lines, it can less efficiently transmit signals. Thus, it is necessary to prevent signals from being transmitted through path P.sub.2. To accomplish this, the part from the break or disconnection position .DELTA. to the path P.sub.2 is cut off, as indicated with an x. Therefore, it is possible to apply signals through the path P.sub.1 of the repair line 100 into the data line D.sub.3 below the disconnected point.
A signal applied through the path P.sub.1 must pass through the intersecting areas a, a' between data lines D.sub.1, D.sub.2 and the repair line 100. These areas a, a', as noted above, act as capacitors and distort picture signals passing through the repair line 100. Particularly, if the size of screen is large and the number of data lines and gate lines increase, the number of intersecting areas and capacitors along the signal path increase and the overall electrostatic capacity grows. Because the length of the repair line 100 is also increased, the resistance increases. Consequently, distortion of the signals transferring through the repair line 100 undesirably occurs.
In order to reduce such signal distortion, the overall electrostatic capacity should be reduced. Since the number of the wires cannot be reduced, the electrostatic capacity of the capacitors formed by the intersecting areas between the repair line 100 and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n should be reduced. Because the electrostatic capacity of these capacitors is proportional to the size of the intersecting area and is inversely proportional to the perpendicular length between the repair line 100 and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n, in order to reduce the electrostatic capacity, the size of intersecting areas should be decreased, or the perpendicular length between the repair line 100 and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n should be increased.
However, because the size of the intersecting areas should be maintained in order to ensure a short-circuit occurs in the intersecting area when laser fusing takes place, it is difficult to decrease the size of the intersecting area and to increase the perpendicular length between the repair line 100 and data lines D.sub.1, D.sub.2, D.sub.3, D.sub.4, . . . D.sub.2n-1, D.sub.2n.